Sail propulsion apparatus



R. A. HERBST,

, SAIL PROPULSION APPARATUS Oct. 31, 1967 2 SheetsSheet 1 Filed May 2, 1966 INVENTOR RICHARD AUGUST HERBST By @W% AT TOR NEV Oct, 1967 R. A. HERBST 1 SAIL PROPULSION APPARATUS Filed May 2, 1966 2 Sheets-Sheet 2 INVENT'OR RICHARD AUGUST HERBST TTORNEV.

United States Patent Ofiice 3,349,741 Patented Oct. 31, 1967 3,349,741 SAIL PROPULSION APPARATUS Richard August Herbst, Farmiugdale, N.Y. (33 Hark Laue, Westbury, N.Y. 11590) Filed May 2, 1966, Ser. No. 546,930 7 Claims. (Cl. 11439) ABSTRACT OF THE DISCLOSURE A demountable sail propulsion apparatus for monohulle-d watercraft which features a base releasably connectible to the hull of the watercraft, a sail-bearing mast supported by the base for propelling the watercraft, and a pair of leeboards pivotally connected to the base on opposite sides of the watercraft, and connected to each other for movement in unison about their pivot points to control the movement of the Watercraft.

This invention relates in general to sail propulsion devices and more particularly to a sail propulsion apparatus which can be used for propelling a wide variety of vehicles across underlying supporting media.

Among the vehicles which can use the sail propulsion apparatus of the instant invention are water craft, including boats and surf boards, land craft such as skate boards, sleds, and iceboats.

Essentially, the invention provides a sail propulsion apparatus including a base means disposed for attachment to the body of the vehicle to be propelled, and hence can be considered as analogous to the outboard motor, except that the apparatus of the invention relies upon natural aerodynamic forces produced by ambient winds for its operation. These aerodynamic forces are received by a sail which is connected to a mast, which in turn is connected to the base means for support thereby.

While the invention can be used with excellent results on small yachts, runabouts, canoes, rowboats, etc. as an auxiliary or emergency propulsion means, practical considerations do limit its most advantageous applications to smaller and light weight vehicles such as surf boards and iceboats. On larger crafts, the use of a detachable sail propulsion apparatus which would be effective presents size and handling problems.

For craft having inherently high roll, pitch and yaw stability, such as for example catamarans and iceboats with relatively large runner spans, a simplified embodiment of the invention comprising a releasably attachable base, a mast mounted to the base, and one or more sails carried by the mast will be satisfactory.

However, in the case of craft such as canoes or surf boards, it is preferable to use an embodiment of the invention featuring a rudder means or leeboard means connected to the base for support thereby and disposed for engagement with the vehicle supporting medium, in these cases water, to stabilize and control the direction of vehicle movement.

As used herein, the term rudder means is intended to designate broadly any means such as a keel, centerboard, daggerboard, skeg, fin, rudder, skid, etc., whether fixed in attitude or adjustable, which can serve for transmitting reaction forces to stabilize the movement of a vehicle in a selected direction, or to change its direction of movement when positioned into engagement with the medium through or over which the vehicle moves. For example, the invention is ideally suited to the propulsion of surf boards because their relatively light weight and low drag enables a comparatively small and portable size mast and sail to be used. Although most surf boards have a small underwater fin for directional stability when used in the conventional manner, the mounting of the sail propulsion apparatus to a surf board introduces additional aerodynamic force moments which ordinarily must be counterbalanced by some means such as Outriggers, leeboards, or other type of rudder means. In the case of surf boards, a sail propulsion apparatus including a pair of oppositely disposed leeboards pivotally connected 0 the base means for movement into and out of the water is most advantageous. Such leeboard movement is preferably eifected in unison by means of a pedal bar connected to both the boards so that the craft operator can easily adjust with his foot the depth to which the leeboards extend into the water.

The base means can be secured to the vehicle body in a variety of ways depending upon the size of the mast and sail combination used. In general, any appropriate conventional releasable fastening means can be used to attach the base means to the vehicle body, including fastening means having one or more members permanently installed on the vehicle body.

However, in the case of surf boards and similar sized vehicles, there need not be any permanently installed fastening members provided on the vehicle body. For example, the base means can be secured to a conventional surf board by one or more flexible tie lines or straps which engage both the surf board and the base means to hold them together. Preferably, the base means is also provided with one or more suction cups disposed in vacuum gripping engagement with the surf board for greater holding power. In addition, or instead of suction cups, the base means can be provided with adjustable clamps disposed to engage the sides of the surf board for securing said base means, and hence the sail propulsion apparatus thereto. Such an arrangement renders the sail propulsion apparatus capable of being easily transferred from one surf board to another.

The mast can be either secured to the base means by a step fitting which provides a permanent type mast-tobase connection, or by a step fitting which provides a releasable type connection.

To permit the sail to be adjustably positioned with respect to the mast and path of vehicle movement, or with respect to the longitudinal center line of a surf board, a boom is expediently provided. Such a boom member is connected to the mast by a conventional gooseneck fitting, as in conventional sail rigging arrangements, to permit the effective area of the sail to be varied by adjusting the intersection angle between the mast and boom, and to permit the sail to be swung around the axis of the mast as needed under various wind conditions.

It should be noted that the sail propulsion apparatus of the invention contemplates the use of a variety of sail rigging arrangements, such as square, gaff, Marconi, spinnaker, and cat type rigs. In any of these sail rigging arrangements, the sail can be provided with either a loose or a fixed fitting on the boom.

The sail propulsion apparatus of the invention is also well suited to the propulsion of iceboats over frozen bodies of water, as well as to the propulsion of land vehicles such as skateboards. In the case of such vehicles being propelled over hard surfaces, the rudder means used in the case of water craft is replaced by skid brake means. For example, instead of using leeboards which are pivoted into and out of the water, a pair of skid brake members are substituted. These skid brakes can be simply bars of metal which are pivotally connected to the base members so as to be capable of being swung into engagement with the ice or road surface underlying the vehicle. Particularly in the case of iceboats, such skid brakes are highly desirable for stopping, as Well as for steering correction purposes, even though the iceboat may be provided with steerable runners. The skid brakes can be either independently operable, or operable in unison, such as for example, by means of a common pedal bar as provided for the leeboards in the case of a surf board installation.

The base means is preferably provided with mounting fittings which will accommodate the interchangeable attachment of either leeboards or skid brakes so as to adapt the same basic sail propulsion apparatus for use with either a surf board or an iceboat.

In the case of those embodiments of the invention adapted for propelling surfboards and equipped with leeboards pivotally connected to their base means, excellent steering control can be achieved simply by manipulating the leeboards about their pivot axes so as to vary the longitudinal position of their centers of pressure with respect to the sail center of pressure. This affords directional control through upsetting the equilibrium of the dynamic forces acting on the surfboard and propelling it. Thus, while the depths to which the leeboards extend into the water can be varied by pivoting the leeboards as required in navigating shallow waters, according to the invention, the pivoting of the leeboards accomplishes a much more important function in navigating, namely that of steering by shifting the leeboard centers of pressure relative to that of the sail or combination of sails.

A properly balanced sailboat or sailing surfboard can theoretically sail close hauled or on a reach along a straight line without the aid of a rudder if the center of pressure of its sail or combination of sails is placed in dynamic equilibrium with the center of pressure of its hull, as determined by the combined geometry of the hull itself, and underwater members such as keels or fins connected thereto. However, such perfect dynamic equilibrium cannot be achieved in general for all sailing conditions and directions, and thus, a controllable unbalancing means, commonly a rudder, is employed to provide directional control. A rudder is ordinarily mounted at the stern of a sailboat for pivotal movement to starboard and to port for respectively turning the sailboat in the same directions. When such a rudder is pivoted to starboard, the center of pressure acting on the hull is effectively shifted toward the stern, and correspondingly, by pivoting the rudder to port, the center of pressure on the hull is shifted toward the bow. Consequently, a stern mounted rudder accomplishes its steering function by the action of a dynamic thrust vector which pushes the stern about until the bow is pointed in the selected direction of travel.

The sail propulsion apparatus of the invention is pro vided with leeboards that can be pivoted about an axis underlying the sail center of pressure to position their centers of pressure either forward of the sail center of pressure for turning to windward, or aft of the sail center of pressure for turning to leeward. When the leeboards are pivoted to position their centers of pressure into underlying relation with respect to the sail center of pressure and in a common transverse section plane therewith, dynamic equilibrium is achieved, and absent any disturbing forces, such as may arise from wind and wave action, the surfboard will sail along a generally straight course. Preferably the leeboards are so constructed and arranged that they assume a normal, control-free position corresponding to such dynamic equilibrium.

One of the advantages of the invention lies in its pivotable leeboard steering control feature, which allows a relatively compact, single-unit type, detachable base means to be used for carrying both the wind propulsion elements, and the steering and stabilization elements as well. Such compactness is made possible because no stern mounted rudder is required, since both steering and stabilization is accomplished by the leeboards which are mounted to a base means that is secured to the surfboard at approximately its mid-length position.

It is therefore, an object of the invention to provide a sail propulsion apparatus which can be releasably secured to a water craft or to a land craft vehicle body for propelling same.

Another object of the invention is to provide a sail propulsion apparatus as aforesaid having means for stabilizing the movement of the vehicle body.

A further object of the invention is to provide a sail propulsion apparatus as aforesaid which can be used in combination with a surf board.

A further object of the invention is to provide a sail propulsion apparatus as aforesaid which can be used to propel an iceboat.

Still another and further object of the invention is to provide a sail propulsion apparatus as aforesaid having means for interchangeably mounting either rudder stabilizing members or skid brake members to adapt it for use with either water craft or vehicles propelled over hard surfaces.

Other and further objects and advantages of the invention will become apparent from the following detailed description and accompanying drawings in which:

FIG. 1 is a perspective view of a sail propulsion apparatus according to a preferred embodiment of the invention illustrated as mounted to a surf board for propelling same.

FIG. 2 is a perspective view of a sail propulsion apparatus according to another embodiment of the invention illustrated as mounted to an iceboat for propelling same.

FIG. 3 is a front elevation view, partly in section, of a portion of a sail propulsion apparatus according to a further embodiment of the invention adapted for use with water craft such as the surf board shown in FIG. 1.

FIG. 4 is a side elevation view, showing details of the sail propulsion apparatus portion of FIG. 3.

Referring now to FIG. 1, the sail propulsion apparatus 10 according to the invention is designed to be releasably attached to the body of a vehicle to be propelled thereby, such as for example, a surf board 11. The propulsion apparatus 10 essentially includes a base platform 12 disposed for releasable attachment to the surf board 11 for support thereby, a mast 13 operatively connected to the base platform 12 for support thereby, and a sail 14 operatively connected to the mast for support thereby and to receive aerodynamic forces for propelling the surf board 11 relative to a body of water (not shown) in which it is buoyantly supported.

To accommodate attachment of the base platform 12 to the surf board 11, said base platform 12 is provided with one or more, in this case four, suction cups 15 attached to its underside and disposed for vacuum-gripping engagement with the upper surface 16 of surf board 11. In general, the number and positioning of the suction cups 15 can be varied, depending upon their individual gripping power and the size of the mast 13 and sail 14 actually used.

To aid in holding the platform 12 onto the top surface 16 of surf board 11, one or more flexible line members, such as a pair of belts 17 passed through slots 19 in platform 12 are provided. Each belt 17 encircles the girth of surf board 11 and has a buckle 18 to facilitate tightening in order to press the suction cups 15 down against the surface 16. In this way, the securing of platform 12 to the surf board 11 will not be dependent upon the holding power of suction cups 15 alone.

The platform 12 is preferably of a width dimension approximately equal to that of the surf board 11, and can be of any convenient length dimension.

The mast 13 is secured to the platform 12 by means of a step fitting 20, which can be simply a sleeve or collar with a bottom flange bolted to the platform 12, or any other suitable fitting (not shown). To permit the mast 13 to be separated from the platform 12 for convenient storage and transportation, one or more cross bolts 21 passing through the fitting 20 and the mast 13 are provided. These cross bolts 21 are preferably loose fitting and are provided with large wingnuts (not shown) to enable mast 13 erection and removal without tools. Any other suitable mast 13 to platform 12 mounting arrangement can be substituted, preferably a type which provides a simple releasable mast 13 mounting.

The invention generally contemplates a sail propulsion apparatus 10, the base platform 12 of which can be releasably attached to any one of a variety of vehicle bodies such as the surf board 11 or the iceboat hull 22 shown in FIG. 2. Such releasable attachment means can be provided in the form of suction cups 15 and/or belts 17 as exemplified by FIG. 1, or can be provided in the form of clamps 23 which grip the sides of the iceboat hull 22. These typical platform 12 to vehicle body securing means can be used interchangeably as desired.

Preferably, each of the oppositely disposed clamps 23 have a gripping face P which matches the contour of the vehicle body, in this case, the hull 22, which has been intentionally shown with a configuration similar to that of the surfboard 11 to better illustrate how the same basic platform 12 can be mounted interchangeably on different vehicles without having to provide any permanent fittings or modifications of the vehicle itself to accommodate installation of the sail propulsion apparatus Each clamp 23 has a pair of studs 24 which extend through the bores 25 of a corresponding block 26 fastened to the platform 12. The clamps 23 can be brought into gripping with sides of the hull 22 by tightening nuts 27 on the studs 24. Preferably, the exterior surfaces of the clamps 23 are faired longitudinally so as to provide a neater appearance, and in the case where such clamps 23 are used to secure the sail propulsion apparatus 10 to a surf board 11, to minimize drag turbulence.

In the case of the sail propulsion apparatus 10 used in combination with a surf board 11, it is preferable to pr0- vide auxiliary means to compensate for aerodynamic moment exerted by the sail 14 and transmitted to the surf board 11 via the mast 13 and platform 12. This can be done quite simply by providing one, or preferably a pair of oppositely disposed leeboard members 28 pivotally connected to blocks 29 secured to the platform 12. The leeboards 28 thus are capable of pivotal movement relative to the platform 12 in respective planes approximately parallel to the mast 13, for movement into and out of the water (not shown) to stabilize the movement of the surf board 11 relative thereto when propelled by the sail 14.

For better balance, a pair of similarly shaped leeboards 28 is used, said leeboards 28 being preferably connected to a pedal bar member 30 for pivotal movement in unison. The leeboards 28 and pedal bar 30 are so constructed and geometrically arranged that said leeboards 28 pivot about a common axis and depend into the water in the approximate position shown in FIG. 1 when in equilibrium, i.e. no external forces acting on the pedal bar 30. This is expedient because the leeboards 28 will assume normal positions of maximum effective keel area andcan be retracted simply by pushing forward and down on the pedal bar 30 to swing it in against platform 12.

While most surf boards have a stern keel or fin 31 for directional stability, it can be appreciated that such fin 31 is normally designed to compensate only for relatively small aerodynamic moments, as when the rider stands up in'a moderate wind. By adding the sail propulsion apparatus 10 to a conventional surf board 11, the aerodynamic moment tending to overturn the surf board 11 is significantly increased, and for this reason the leeboards 28 are recommended. For generally acceptable stability, the'leeboards 28 are shaped to have a combined total Wetted area equivalent to a 37.5 to 1 sail 14-to-total leeboard area ratio. By using a pair of leeboards 28, each can be made physically smaller than would be possible with a single leeboard, for a given sail 14 area ratio.

As can be appreciated by the artisan, the same type of stability provided by the leeboards 28 can be achieved by substituting either a single dagger board (not shown) or a pair of oppositely disposed dagger boards (not shown) which are slidably movable up and down in guides (not shown) attached to the platform 12.

While a variety of sail n'ggings can be used in the sail propulsion apparatus 10, the simple sail 14 rigging ar rangement shown in FIG. 1 will be generally satisfactory. In this arrangement, the foot of the sail 14 is secured to a boom 32 which is pivotally connected to the mast 13 as by a conventional gooseneck fitting 33, and the luff of the sail 14 is secured to the mast 13 along the length thereof. Directional control of the surfboard 11 movement can be obtained by swinging the boom 32 about the mast 13 to adjustably position the sail 14 relative to the surfboard longitudinal center line.

The pivotally connected leeboards 28, provide a capability for positive steeringcontrol Without the aid of a sterm-mounted rudder (not shown) in spite of the fact that such leeboards 28 are constrained to move in planes parallel to the plane defined by the mast 13 and longitudinal centerline of the surfboard 11. This steering control is achieved by pivoting the leeboards 28 to position their centers of pressure CL either forward or aft with respect to the sail 14 center of pressure CS. When the leeboards centers of pressure CL are forward of the sail center of pressure CS, the surfboard 11 will be turned to windward, and conversely, the surfboard 11 can be turned to leeward by pivoting the leeboards 28 into a position where their centers of pressure CL are aft of the sail center of pressure CS.

As shown in FIG. 1, the leeboards 28 are connected to the blocks 29 to pivot about a common axis transverse to the longitudinal centerline of the surfboard 11 and lying in a transverse section plane approximately passing through the sail center of pressure CS. The leeboards 28 can thus be pivoted to position their centers of pressure CL at various points along an arcuate path P, as for example to a typical position where their centers of pressure CL are a distance X aft of the sail center of pressure CS.

The leeboards 28 are preferably so constructed and balanced, taking into account their buoyancy, that the surfboard 11 and sail propulsion apparatus 10 combination will assume a normal, control-free. straight-line course of dynamic equilibrium. This can be accomplished by constructing and mounting the leeboards 28 so that their centers of pressure CL coincide with the dynamic equilibrium point X0 on the path P. This point X0 lies in substantially the same transverse plane as the sail center of pressure CS and pivot axis of the leeboards 28.

From the geometry shown in FIG. 1, it can be appreciated that when the leeboard centers of pressure CL are shifted either forward or aft of the point X0, the dynamic forces acting on said leeboards 28 and the sail 14 will produce a couple or moment about an axis parallel to the mast 13 tending to turn the surfboard 11 to windward or leeward respectively. When the centers of pressure CL are coincident with the X0 position, the magnitude of such turning moment will be theoretically zero, and hence X0 is the position for dynamic equilibrium. In general, within the limitation that the immersed area of the leeboards 28 remains substantially constant, the magnitude of the turning moment, and thus the turning rate increases as their centers of pressure CL are positioned further from the equilibrium point X0.

Directional control over surf board 11 movement can also be obtained by the rudder means 34 shown in FIGS. 3 and 4, which functions to provide a pair of keel-rudder members 35 that are generally platelike and rotatable in unison about parallel axes which can be tilted in unison Within respective planes parallel to the mast 13. Thus, the members 35 have two degrees of freedom rotary movement capability, one for retracting and lowering them into the water (not shown), and the other for rotating them as rudders. The rudder means 34 is basically an extension of the principles of leeboard 28 mounting design of FIG. 1, with modifications to permit the members 35 to function both as rudders and as keels. Each member 35 is secured to the end portion of a shaft 36 which extends through a tubular holder 37 and is free to rotate relative thereto. The holders 37 have affixed outrigger arms 38 which are journaled to blocks 39 for rotation relative thereto and relative to the platform 12 about a common axis X which is substantially at right angles to surf board 11 longitudinal center line. This enables the members 35 to be retracted and lowered into the water (not shown). At the upper end of each shaft 36, a crank 40 is secured, and the outer ends of the cranks 40 are pivotally connected to a link bar 41, which when pushed or pulled sideways with respect to the platform 12 rotates the members 35 in unison about their respective shaft 36 axes to function as rudders when the holders 37 are swung into a position which immerses the members 35. The holders 37 are connected to a common pedal bar 30a, which functions similarly to the pedal bar 30 to swing said holders 37 in unison. If desired, the pedal bar 30a can be omitted since the link bar 41 can also be used for accomplishing the same purpose, although the pedal bar 30a serves to provide a somewhat greater rigidity in shaft 36 alignment.

In general, the length of the outriggers 38, the maximum tip radius of the members 35, and the relative widths of the platform 12 and surf board 11 determines whether or not it will be necessary to first manipulate the link bar 41 to position the members 35 parallel to the surf boa-rd 11 center line before swinging the holders 37 to raise or lower the members 35, in order to clear the edges of the surf board 11. By making the Outriggers 38 long enough, to taking into account the spacing of the blocks 39 with respect to the surf board 11 edges, sufficient surf board 11 edge clearance can be obtained to permit raising and lowering of the members 35 regardless of the position of link bar 41.

One of the advantages afforded by the rudder means 34 is that it provides the sail propulsion apparatus and surf board 11 combination with an improved capability for maneuvering under sail in shallow water, because rudder control is still available even though the members 35 are partly retracted.

For easy storage and transportation, the blocks 39 can be constructed so as to be separable along a diametral plane 42 through their journal bores 43 to permit the Outriggers 38 wit-h the assembled holders 37, shafts 36, members 35, etc. to be lifted out and replaced. This can be achieved simply by providing a hinge 44 connection between the upper and lower block portions 45 and 46 respectively and a releasable locking means 47 opposite to the hinge 44 for securing said block portions 45 and 46 together when the rudder means 34 assembly is mounted to the blocks 39. The locking means 47 is opened to permit the block portions 45 and 46 to be swung apart as when removing or replacing the rudder means 34 assembly. Such arrangement simplifies the construction of the holders 37 since their outriggers 38 can be simply provided each with a pair of collar portions 48 spaced apart by a distance approximately equal to the thickness of the blocks 39, with the axial portions of said outriggers 38 between collars 48 being approximately equal in diameter to the journal bores 43 in blocks 39. In fact, only one similarly disposed collar 48 need be provided on each outrigger 38 since the blocks 39 are secured to the platform 12. Since the Outriggers 38 are simply laid into the journal bore halves of the lower block portions 46, and the upper block portions 45 are swung over and locked, no special tools are needed to remove and replace the rudder means 34- assembly.

The invention is by no means limited in its application to water craft or surf boards 11, and as exemplified by FIG. 2, can be utilized in combination with practically any vehicle body adapted for movement over a substantially solid supporting surface, such as for example, an iceboat hull 22.

Basically, the same sail propulsion apparatus 10 is releasably secured to the iceboat hull 22 by means of clamps 23 which grip the sides thereof. The aerodynamic forces received by the sail 14 propel the iceboat hull 22 over the surface of a frozen body of water (not shown) on its supporting forward and rear runners 49 and 50 respectively. The rear runners can be either secured to the hull 22 by a fixed short outrigger beam 51 or by a free pivoted outrigger beam, similar to the pivoted outrigger beam 52 which carries the forward runners 49 and which can serve for steering via a control bar 53 with cables 54 secured to said outrigger beam 52.

For an iceboat hull 22, the leeboards 28 or rudder means 34 used in conjunction with surf board 11 are replaced by oppositely disposed skid brake members 55 connected to the base platform 12 and disposed for movement relative thereto from a retracted position into a position of engaging contact with the ice surface to decelerate the hull 22. Each skid brake 55 is pivotally connected to an outrigger beam 56 secured to the platform12, and thus can be swung up out of contact with the ice or swung down into engaging contact therewith simply by respectively pushing forward on its upper extension 57 or pulling backward thereon. For convenience, the skid brakes 55 are connected to a bar member 58 for pivotal movement in unison, but if desired, the bar 58 can be omitted to allow independent control of each skid brake 55 for steering compensation purposes. Preferably, the skid brakes 55 are disposed for pivotal movement in planes mutually parallel and parallel to the mast 13.

As can be appreciated by the artisan, the sail propulsion apparatus according to the invention herein is susceptible of numerous modifications and variations to adapt it for the expediencies of particular applications. However, the invention is intended to be limited only by the following claims wherein I have endeavored to claim all inherent novelty.

What is claimed is:

1. A sail propulsion apparatus which comprises a base means releasably connectable to the upper hull portion of a monohulled watercraft, a mast connected to said base means for support thereby in an elevated position above said hull and centrally located with respect to the sides thereof, a sail connected to said mast for support thereby and to receive aerodynamic forces for propelling said watercraft, and a pair of leeboards pivotally connected to said base means on opposite sides of said hull, bar means connecting the upper ends of said leeboards for movement relative to said base means and hull in unison about their pivot points to control the movement of the watercraft.

2. The sail propulsion apparatus according to claim 1 wherein said base means is disposed for releasable connection to the upper portion of a surfboard.

3. The sail propulsion apparatus according to claim 1 including at least one suction cup operatively connected to the base means and disposed for vacuum gripping engagement with the upper hull portion to releasably secure said base means thereto.

4. The sail propulsion apparatus according to claim 1 including at least one clamp member operatively connected to the base means and disposed for operative engagement with the watercraft hull to releasably secure said base means thereto.

5. The sail propulsion apparatus according to claim 3 including at least one flexible line member disposed for operative engagement with said base means and with said watercraft hull to hold said suction cup in vacuum gripping engagement therewith.

6. The sail propulsion apparatus according to claim 1 wherein said leeboards are pivotally connected to said base means along a common pivot axis extending transversely with respect to the base means and disposed in underlying relation to the sail center of pressure, and wherein the centers of pressure of said leeboards are disposed for movement into positions forward and aft of the sail center of pressure when said leeboards are pivoted into re- 5 spectively corresponding positions about said axis.

7. The sail propulsion apparatus according to claim 6 wherein said leeboards are disposed to assume normal positions wherein their centers of pressure are in underlying relation to the sail center of pressure.

References Cited UNITED STATES PATENTS Brodie 114-39 Hansen 114-39 Kibby 114-39 X Owens 9-310 Kiefer 11439 X MILTON BUCHLER, Primary Examiner. T. M. BLIX, Assistant Examiner. 

1. A SAIL PROPULSION APPARATUS WHICH COMPRISES A BASE MEANS RELEASABLY CONNECTABLE TO THE UPPER HULL PORTION OF A MONOHULLED WATERCRAFT, A MAST CONNECTED TO SAID BASE AND HULL AND CENTRALLY LOCATED WITH RESPECT TO THE SIDES AND HULL AND CENTRALLY LOCATED WITH RESPECT TO THE SIDES THEREOF, A SAIL CONNECTED TO SAID MAST FOR SUPPORT THEREBY AND TO RECEIVE AERODYNAMIC FORCES FOR PROPELLING SAID WATERCRAFT, AND A PAIR OF LEEBOARDS PIVOTALLY CONNECTED TO SAID BASE MEANS ON OPPOSITE SIDES OF SAID HULL, BAR MEANS CONNECTING THE UPPER ENDS OF SAID LEEBOARDS FOR MOVEMENT RELATIVE TO SAID BASE MEANS AND HULL IN UNISON ABOUT THEIR PIVOT POINTS TO CONTROL THE MOVEMENT OF THE WATERCRAFT. 